1. Field of the Invention
The present invention relates to a multibeam scanning optical device and an image forming apparatus using the same. In particular, the present invention is preferably applied to an image forming apparatus such as a digital copying machine, a laser beam printer, or a multi-function printer which synthesizes a plurality of light beams emitted from a plurality of light sources to simultaneously scan a plurality of lines in parallel.
2. Related Background Art
Conventionally, as a technique for increasing processing speed of a scanning optical device for a digital copying machine, a laser beam printer, a multi-function printer, or the like, there has been known, for example, a “multibeam scanning optical device” for realizing the increase in processing speed by focusing a plurality of light beams on a surface to be scanned at a predetermined gap in a sub-scanning direction and forming a plurality of scanning lines simultaneously according to scanning by an optical deflector.
Here, as methods of generating a plurality of beams, for example, there are a method of synthesizing a plurality of light beams emitted from a plurality of light sources so as to be emitted in a substantially identical direction using beam synthesizing means and scanning a plurality of lines, and a method of using a monolithic multibeam laser in which a plurality of light emission points are integrated at a very small gap.
In the latter method, a positioning accuracy is high because the light emission points are integrated at the very small gap. Thus, a scanning lines gap is free from deviation due to relative deviation among beams. However, since thermal and electrical interference of the light emission points occurs due to the integration thereof, it is difficult to stabilize a beam quality, and there is limitation in the number of light emission points which can be integrated.
On the other hand, in the former method, a plurality of light beams emitted from a plurality of independent light sources can be synthesized in proximity to each other. Thus, the same effect as integrating the light sources at a very small gap can be obtained. This method is advantageous in that there is no limitation in the number of light sources and increase in processing speed can be realized by making scanning multiple. However, in the multibeam scanning optical device, a gap in a sub-scanning direction of scanning lines for scanning a surface to be scanned has to be fixed in order to realize satisfactory optical scanning. In particular, in a synthesizing optical system, if a light source and a collimator lens relatively deviate in the sub-scanning direction, or if a change in posture such as relative inclination of a plurality of pairs of light sources and collimator lenses in the sub-scanning direction occurs, gaps of a plurality of scanning lines fluctuate, and an image is deteriorated.
Therefore, the synthesizing optical system requires a mechanism (adjusting means) for adjusting a gap of scanning lines to a predetermined scanning lines gap with a sufficient adjustment accuracy in initial adjustment. In addition, the synthesizing optical system also requires a mechanism for maintaining the scanning lines gap in a predetermined range with respect to a change over time of the scanning lines gap due to environmental variation such as mechanical vibration or temperature rise.
Various multibeam scanning optical devices for solving such problems have been conventionally proposed.
FIG. 10 shows a conventional multibeam scanning optical device. The multibeam scanning optical device causes two beams from two perpendicular directions to be incident in a beam splitter 89 serving as beam synthesizing means, transmits one beam through all sides of the beam splitter 89, and causes the other beam to reflect on one side. Then, the multibeam scanning optical device rotates the beam splitter 89 around an arbitrary axis to thereby deflect a beam emitted from the beam splitter 89, and provides an angular difference between a reflected beam and a transmitted beam in a sub-scanning section (e.g., see Japanese Utility Model Application Laid-open No. S61-196717).
In other words, in Japanese Utility Model Application Laid-open No. S61-196717, the multibeam scanning optical device makes angles in a sub-scanning direction of two synthesized beams to be incident in cylindrical lenses (not shown) different from each other to thereby cause the synthesized beams to focus on a surface to be scanned apart from each other so as to adjust a scanning lines gap to a predetermined scanning lines gap.
On the other hand, in another multibeam scanning optical device, a plurality of light sources are arranged so as to be substantially parallel to each other. The multibeam scanning optical device transforms light beams emitted from the respective light sources into substantially parallel light beams using corresponding collimator lenses, and generates synthesized beams using a composite prism serving as beam synthesizing means in which a parallel prism and a triangular prism are stuck together (e.g., see Japanese Patent No. 2942721).
In Japanese Patent No. 2942721, the synthesized light beams are set to have a predetermined inter-beam angle on a plane shared by the synthesized light beams. This inter-beam angle provides an angular difference to parallel light beams emitted from the respective collimator lenses by slightly deviating one light source from an optical axis of the collimator lens. The composite prism plays a role of synthesizing means which emits the plurality of beams in proximity to each other.
In addition, the light sources, the collimator lenses, and the composite prism are integrated as an optical unit and are adapted such that synthesized light beams always have a predetermined inter-beam angle on a plane shared by the synthesized light beams. Adjustment of an interline gap is performed by rotating and adjusting the optical unit around an optical axis such that the inter-beam angle has a component within a sub-scanning surface. Since a plurality of beams are incident in the cylindrical lenses at different angles in the sub-scanning direction, the beams are focused in the vicinity of a deflected surface apart from each other in the sub-scanning direction and are focused again on the surface to be scanned at a predetermined focus magnification by a scanning lens, and adjusted to a predetermined scanning lines gap.
In the multibeam scanning optical device of Japanese Patent Laid-open No. 2942721, the plurality of light sources are arranged substantially in parallel with each other, and therefore, the plurality of light sources and collimator lenses are easily held as a common member. It can be said that, compared with the multibeam scanning optical device in Japanese Utility Model Application Laid-open No. S61-196717, the multibeam scanning optical device of Japanese Patent No. 2942721 is resistant to a relative change of posture among light sources due to vibration or environmental variation. In addition, since the plurality of light sources are arranged in parallel with each other, the light sources can be arranged on a common circuit substrate to be driven. Thus, reduction in cost can be expected by decreasing the number of components.
Incidentally, the methods of adjusting a scanning lines gap described in Japanese Utility Model Application Laid-open No. S61-196717 and Japanese Patent No. 2942721 utilize the principle that a plurality of light beams are incident in cylindrical lenses at different angles in a sub-scanning section.
Usually, an angle of a light beam incident in a cylindrical lens in a sub-scanning section has a very high sensitivity with respect to a scanning lines gap. Therefore, in Japanese Patent No. 2942721, the optical unit once generates an angular difference in a main scanning surface, and then the entire optical unit is rotated around an optical axis such that a very fine angle is formed in the sub-scanning surface so as to reduce the sensitivity.
However, if the sensitivity is reduced in order to adjust the scanning lines gap to a predetermined scanning lines gap with a high accuracy, the adjustment range is reduced. In other words, a scanning lines gap error, which occurs in a range of 0.1 mm to 1 mm due to a component error or a low assembly accuracy, cannot be absorbed by the adjustment methods. Therefore, conventionally, for example, a long time is required for highly accurate processing of components and accurate adjustment for assembly in order to suppress a scanning lines gap error which occurs in an assembly process, resulting in an increase in manufacturing cost.